1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on both sides of a recording medium or an image on one side of a recording medium in a multiplex fashion, and, more particularly, to an image forming apparatus for forming an image on both sides or in a multiplex fashion, which is capable of properly straightening curls of a recording medium and increasing the image processing speed.
2. Description of the Related Art
Hitherto, an image forming apparatus capable of automatically forming an image on both sides of a recording medium has been provided. In such an apparatus, after image formation on one side of a recording medium, such as paper, the recording medium is sent once again to the front of the image forming section where an image is formed on the reverse side, followed by discharge of the recording medium with images formed on both sides. In the construction of many of these apparatuses, the recording medium, after fixing, is bent about 90 degrees from the horizontal in the vertical direction, and then further bent by about another 90 degrees from the vertical to the horizontal direction. Since the recording medium, which is still hot immediately after fixing, is conveyed through a bent path, the recording medium ordinarily curls downward, that is, the recording medium is curled with the image on one side thereof facing outward. Such a curl becomes a great obstacle when conveying and loading the recording medium onto an intermediate tray. Therefore, the recording medium is, ordinarily, made to pass through a section which functions to straighten the curl.
In recent years, however, there has been an increasing demand for higher productivity during image formation on both sides of a recording medium. In such a situation, an increase in the performance speed of the image forming sections, namely the optical system, the developing system, and fixing system, must be achieved by implementing better materials and component parts, thereby increasing apparatus costs. Accordingly, to increase productivity during image formation on both sides of a recording medium, the sheet feed speed at the two-side image forming section that is not related to the image-forming section, that is, the section which conveys the recording medium to the sheet feed section after the recording medium has left the fixing device, is made greater than the sheet feed speed at the image forming section.
In such a case, however, the recording medium is ordinarily conveyed at a greater speed at the section capable of straightening curls than at the image forming section. In general, curl is more effectively straightened, when the recording medium takes a longer time to pass the section capable of straightening curl. Accordingly, since the recording medium takes only a short time to pass the curl straightening section, the amount of curling is excessive. As a result, jamming or oblique movement occurs when the recording medium is being loaded onto the intermediate tray or being conveyed.